Scrubbers

Scrubbers are an apparatus that cleans the gases passing through the smokestack of a coal-burning power plant. Due to Clean Air Act regulations, most scrubbers in U.S. coal plants are used to remove sulfur emissions from coal and lessen the formation of acid rain. In this use, scrubbers are large towers in which aqueous mixtures of lime or limestone absorbers are sprayed through the emissions, known as flue gases, exiting a coal boiler. The lime/limestone absorbs some of the sulfur from the flue gas. The advantage of scrubbers is that sulfur emissions are lower than in standard coal plants, but the plants are more complex and require more maintenance.

Sulfur dioxide and coal
Coal-fired power plants are the largest human-caused source of sulfur dioxide, a pollutant gas that contributes to the production of acid rain and causes significant health problems. Coal naturally contains sulfur, and when coal is burned, the sulfur combines with oxygen to form sulfur oxides. Although natural processes like volcanic eruptions, biological decomposition, and forest fires also produce sulfur dioxide, emissions caused by human activity far exceed natural production in developed countries.

History of Scrubbers
Although some scrubbers had been built in Great Britain in the 1930s, it wasn't until 1967 that the first full-scale scrubber at a coal-burning power plant began operating in the United States. The first scrubbers converted sulfur into a sludge-like waste product, however new treatment processes can produce a dry powder, which have been used to make wallboard and other commercial purposes.

U.S. Regulations
The Clean Air Act of 1970 imposed more stringent pollution control requirements on coal-fired power plants, and many companies attempted to comply by building taller smokestacks to disperse the flue gases over a wider area, or by burning lower sulfur coal (coal that developed primarily in fresh water, as opposed to high sulfur coal that formed primarily in salt water swamps). However, the 1970 Act also accelerated research on a “flue gas desulfurization unit”, or “scrubber.” Rather than trying to remove sulfur from coal before it was burned by washing it, which had little effect, scrubbers aimed at the “back end” of a power plant, trying to remove sulfur in the form of sulfur dioxide (or SO2) from the flue gas exiting a coal boiler.

In 1977 Congress passed a new Clean Air Act essentially mandating that all new coal-fired power plants install scrubbers, although old coal plants were grandfathered in and exempted. Instead, the New Source Review (NSR) amendment to the CAA required that, before any expansions, older industrial facilities submit an EPA assessment to see if they were required to also install modern pollution control technologies.

In November 2009, the EPA proposed new limits on sulfur dioxide, the first time since 1971 that the agency has recommended tightening controls on SO2 to protect public health. The old limits measured sulfur dioxide averages over 24-hour and one-year periods. The new rule would require one-hour measurements, such that a spike of emissions above a new limit — between 50 and 100 parts per billion in one hour — would no longer be acceptable.

The EPA approximates that CAA measures to comply with SO2 emission standards have resulted in a 40% decrease in SO2 emissions from 1990 levels.

Plants with Scrubbers
According to data from the Energy Information Administration, the following proportions of coal-fired power plants with capacity over 100 MW had SO2 scrubbers in 2005:

It is possible that some coal-fired plants with SO2 scrubbers did not report their scrubbers to the EIA, and thus that the above table overstates the number of plants without SO2 controls. However, out of 257 U.S. coal-fired power plants which produced more than 2,000 GWh of power in 2006, 86 had SO2 emissions that were higher than 10 lb/MWh – compared with an average of 1 lb/MWh for coal plants with state-of-the-art SO2 scrubbers. We can surmise that these 86 plants almost certainly have zero or extremely minimal SO2 scrubbers, or have SO2 scrubbers that were not functioning in 2006.

Disposal
The waste byproducts from scrubbers require some means of disposal. The EPA estimates that only 30% of these materials are recycled, most often for commercial building products, while the other 70% is deposited in landfills. In Ohio, for example, the state's EPA recently approved a landfill for a FirstEnergy plant in Jefferson County, expected to accumulate between 1.2 million and 1.6 million tons of coal sludge per year. Another landfill for AEP’s Gavin Power Plant took on 3 million tons of scrubber sludge and coal ash in 2007.

The byproduct from the bottom of a scrubber system is called gypsum waste, similar to wet sand. This byproduct is the principal component in wallboard, chalk and plaster.

Decay found in new scrubbers
The Electric Power Research Institute (EPRI), which is funded by utility companies, is investigating reports of "aggressive" corrosion in scrubbers across the nation. There are about 360 operating scrubbers at U.S. power plants. Research Institute officials are focused on 166 scrubbers installed since 2006. As many as 70 are made of a type of stainless steel that appears particularly vulnerable to corrosion. A scrubber can hold as much as 1 million gallons of lime slurry, a solution that captures sulfur compounds in hot power-plant smoke before it goes up the stack. Although no scrubber has "failed," utility officials say they want to know why some are corroding. EPRI said it could take as long as two years to identify a root cause of the corrosion and find a solution.

American Electric Power installed a pollution control scrubber at its Cardinal Plant along the Ohio River in 2007, as part of a 2007 federal air-pollution lawsuit settlement. The scrubber was supposed to last 25 years, but a 2008 inspection found that something was eating through its steel wall, with some areas corroded all the way through. Central Ohio AEP customers already are paying for the scrubbers, as state regulators permitted the company to increase base fees as much as 7 percent in 2009, 6 percent in 2010 and another 6 percent in 2011. Estimates kept by the Public Utilities Commission of Ohio show the average Columbus residential electricity bill for the month of June rose from $87.90 in 2008 to $95.21 this year.

In addition to Cardinal, AEP found corrosion in new scrubbers at its Conesville Power Plant in Coshocton County and at its Mountaineer Plant and Mitchell Plant along the Ohio River in West Virginia. The company spent $1.7 billion to install five scrubbers at those four plants, said Melissa McHenry, a company spokeswoman. AEP negotiated a confidential settlement with a contractor, Kansas-based Black & Veatch, to address corrosion at its Conesville and Cardinal plants.

Duke Energy found corrosion in two Miami Fort Station scrubbers in Hamilton County, which cost $365 million. Duke Energy spokeswoman Erin Culbert said the company, so far, has spent more than $5 million on short-term repairs. Ohio-based FirstEnergy found initial signs of corrosion in three new scrubbers - only seven months old - at its W.H. Sammis Plant along the Ohio River in Jefferson County. It cost $1.8 billion to install scrubbers and other pollution filters there.

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 * Existing U.S. Coal Plants
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 * Sulfur dioxide and coal
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 * EPA Coal Plant Settlements